The present invention relates to a sound processing apparatus, and more particularly to a sound processing apparatus used in a game computer system.
Traditionally, in a computer system, sound is produced from waveform data, which is generated by a computer program based process; however, the quality of the sound has been low. For that reason, sound data (analog signals) now are converted into digital signals so that the sound waves may be synthesized by an arithmetic operation.
In general, a game computer use a programmable sound generator (PSG), which is small in size and capacity. In the PSG, wave data supplied by a CPU are modulated in amplitude or frequency in order to generate a sound wave. The PSG may generate simple waves to intentionally produce noise. According to the PSG, it is easy to control the output sound; however, it is difficult to generate a variety of sounds.
For A/D conversion, a pulse code modulation (PCM) method is used, by which an analog signal is sampled at predetermined intervals, the sampled data are quantized, and then, are transformed into binary data.
According to a difference PCM (DPCM) method, the difference of the next two sampled data is quantized so that the amount of output data is reduced. Further, according to an adaptive difference PCM (ADPCM) method, the quantizing process is performed at a short pitch when the next two sampled data have a great difference, and on the other hand, the process is performed at a long pitch when they have a small difference. As a result, the output data may be more compressed.
The PCM and ADPCM data are compatible with each other by compression and extension processing, which is performed based on conversion between scale value and scale level, and between the ADPCM data, the changing amount and changing level of the data.
In a game computer, ADPCM sound data stored in an extra recording device are read by a CPU, and the data are extended by an ADPCM decoder in accordance with a scale value and a scale level, so that the original sound is reproduced. The ADPCM decoder contains a synchronizing signal generating circuit, which generates a transmission rate using a crystal resonator. The PCM data are reproduced in accordance with the transmission rate.
Recently, the game computer has become provided not only with a sound source such as PSG and ADPCM controlled by the CPU, but also an external audio device to realize high quality sound reproduction. For example, in a game computer using a CD (compact disk) as recording medium, a CD player is directly used as the PCM sound source to generate high quality sound.
Generally, the sound data are controlled in volume by a volume control circuit, and output sound is supplied through a mixer circuit. Basically, two types of volume control circuits, analog and digital types, are used in game computers. According to the volume control circuit of the analog type, volume of output sound is controlled by a voltage signal. The volume control circuit of the digital type includes a D/A converter, in which a conversion ratio is changed for each bit. An attenuation amount (N) of a volume control circuit is given by the equation "N(Db)=log (I.sub.1 /I.sub.0)," where I.sub.1 and I.sub.0 represent levels of input and output signals, respectively. Most volume control circuits include registers holding values for specifying attenuation values of sound data.
FIG. 1 shows the relation between register values and attenuation values for a volume control circuit contained in a conventional sound processing apparatus. As shown in this table, an attenuation range of 12 dB is divided into eight levels, in which the first four values are set to have 1 dB differences from each other, and the last four values are set to have 2 dB differences from each other. The attenuation levels -1 dB, -2 dB, -3 dB, -4 dB, -6 dB, -8 dB, -10 dB and -12 dB correspond to register values 7, 6, 5, 4, 3, 2, 1 and 0, respectively.
In the conventional volume control circuit, the difference values of attenuation are not constant, because it is difficult to divide the total attenuation level (attenuation range) constantly, especially at the maximum and minimum levels. According to the conventional volume control circuit, the register values are determined to correspond to the attenuation levels one-to-one, and therefore, it is difficult for an operator to adjust the sound volume to desired levels.
A conventional game computer contains a sound source chip for sound processing. Most sound source chips include sound sources for generating sound, and volume control circuits for controlling volume of the sound supplied from the sound sources. The volume control circuits are structured to be adapted to the characteristics of sound to be reproduced and the performances of amplifiers, speakers and the like.
When an external volume control circuit is provided with the game computer to realize high performance sound processing, a controller circuit is required to be built in the system to control the external volume control circuit.
Recently, a plurality of sound sources are employed in a game computer to treat a variety of sound, to produce special sound and music effects. However, such a high performance system is expensive for a game computer, because plural volume control circuits having different performances must be controlled properly.
Generally, a plurality of sound sources, such as PSG and ADPCM, are contained in different sound chips individually, and output sound of the sound chips are also supplied to an external mixer circuit.
When an external sound source, such as a CD player with no volume controller, is employed in the system, an extra volume control circuit is necessary to be contained in the system to control volume of the player. As described above, when a plurality of sound sources are employed in a system, the circuitry in the system becomes complicated, and as a result, the cost of the system becomes high.
In the conventional game computer, volume and output controlling by the volume and mixer circuits are performed in accordance with values held in internal registers built in the volume control circuits, the register values being set by a CPU. According to the conventional system, the circuitry in the system becomes complicated because the CPU controls the sound chips individually.